Mammographic signs of early cancer include calcifications, small masses (densities), neo-densities and architectural distortions. Screening mammography aims at revealing these signs and is highly sensitive and specific. However, still a notable fraction of mammography produces false positive or false negative results These outcomes most occur in the thick and/or dense breast. False positive results may result in an unnecessary additional imaging and biopsy. False negative results, which occur sometimes even when a cancer is palpable, have a severe adverse effect due to delayed biopsy and delayed cancer diagnosis.
Presence of calcifications usually helps to detect breast pathology. Although calcifications demonstrate well-pronounced contrast in mammograms, their detection threshold size is a function of x-ray tube focal spot size, radiographic technique factors including radiation quality, signal-to-noise ratio and exposure time, as well as function of detector and display devices. Even though, in early stages of breast disease microcalcifications are present in approximately half of the cases, they are not apparent on regular screening mammographs before reaching the detection threshold size, which is typically around 100 μm. This especifically applies to punctuate calcifications
Some of the inherent limitations of the x-ray tube based screen-film mammography include:    relatively large focal spot (typically nominal ˜100 μm);    varying effective focal spot size across the imaged breast;    suboptimal (below 13 lp/mm) spatial resolution in the upper quadrants for breast thickness above 6 cm:    off-focal radiation;    a relatively low output (especially with microfocus);    a suboptimal spectral characteristic of x-rays for imaging dense fibroglandular tissue and/or thick breast;    restricted latitude of film-screen mammography, and    restricted contrast of film-screen mammography.
Regarding the two last restrictions, film-screen mammography utilizes film as a recording medium to properly record and display relatively narrow dynamic range of x-ray exposures. This should be contrasted with specially designed detectors that can correctly record four orders of magnitude of x-ray exposure. This information can be subsequently displayed on specially designed monitors. Moreover, since film-screen mammography aims at obtaining very high contrast images, the slope of the optical density/x-ray exposure curve is very high. As a result, only a very limited range of x-ray exposure is acceptable If exposure is too high or too low, it will produce exceedingly high or low optical density thus rendering the image not useful clinically.
While the two last restrictions can be alleviated to a great extent by digital mammography, the other limitations stem from the inherent limitations of x-ray tube and are unlikely to be overcome in the framework of this technology.
New technology for generating x-rays had emerged in recent years It relies on emission of x-rays from laser-produced plasma (LPP). This phenomenon occurs when a visible or infrared laser beam is focused onto the surface of solids or liquids If the optical power density exceeds a material dependent threshold value, continuous bremsstrahlung and characteristic x-ray emission lines result. However, initially very expensive and large laser systems were required to obtain the required optical power and the LPP x-ray sources were rather large (100 μm-1 mm). The invention of chirped-pulse amplification (CPA) in the late 1980's allowed achievement of high optical power density delivered to the target by the laser beam from compact and significantly cheaper table-top terawatt ultra-fast lasers.
The feasibility of CPA lasers for mammography and angiography has been demonstrated in both “High Magnification Imaging With a Laser-Based Hard X-ray Source”, IEEE Journal of selected topics in Quantum Electron, Special issue on laser in medicine, 5, 911-915 (1999), by J. Yu, Z. Jiang, J. C. Kieffer, A. Krol. and “Laser-Based microfocused X-ray Source for Mammography faisability Study”, Journal of Medical Physics, 24, 725×732 (1997), by A. Krol, A. Ikhlef, J C Kieffer, D. Bassano, C. C. Chamberlain, Z Jiang, H. Pepin, S. C. Parsad. Data in these publications shows the ability to obtain focal spot size of the order of 10 μm necessary to perform high spatial resolution mammographic imaging and confirm system ability to produce x-ray spectra from a number of different target materials, including Mo, Rh, Ag, In and, Sn with characteristic emission energies spanning 17 3 keV to 28.5 keV.
Even though LLP x-ray source created by CPA lasers can be very small (10 μm or less) and bright, with peak power many order of magnitude higher than conventional x-ray tubes, a drawback of CPA laser sources from the prior art has been their low average power However, in recent years, significant progress has been made in this respect 20 W average power CPA lasers are presently available, and a 50 W CPA laser is in the design stage.
Finally, there have been studies on sub-picosecond laser-solid-matter interaction devoted to investigation of x-rays generation, from the soft x-rays up to the very hard x-ray emission in the MeV range. However, nobody attempted to simultaneously control the emitted x-ray spectrum, the x-ray source size and conversion efficiency from laser to x-rays.
Method and system for generating microfocused laser-based x-rays allowing to simultaneously optimize the x-ray source size, its spectral distribution, and the conversion efficiency in the 17 3-28.5 keV range is thus desirable.